<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script src="https://cdn.jsdelivr.net/npm/vue@2.6.10/dist/vue.js"></script>
</head>
<body>
    <div id="app">
        姓：<input type="text" v-model="firstName"><br><br>
        名：<input type="text" v-model="lastName"><br><br>

        全名:<span>{{fullName}}</span>
    </div>
    <script>
        Vue.config.productionTip = false;

        const vm = new Vue({
            el: "#app",
            data() {
                return {
                    firstName: "",
                    lastName: "",
                    fullName:""
                }
            },
            // 计算属性，不可开启异步任务，不能写定时器
            // computed: {
            //     fullName() {
            //         return this.firstName + "-" + this.lastName
            //     }
            // }

            // watch实现
            watch: {
                firstName(val){
                    this.fullName = val  + this.lastName
                },
                lastName(val){
                    this.fullName = this.firstName + val
                }
            }
        })
    </script>
</body>
</html>